Dry vernal pools are still pools!

They may be hard to spot, but they still need to be protected.

Dry vernal pool.
It’s still a pool! Look for flattened, gray or dark brown leaf litter when water isn’t present. Photo by James Ford, Grand Valley State University.

Vernal pools are small (less than 2.5 acres) wetland depressions in forests located throughout Michigan. They’re called vernal pools because they are temporary, usually holding water only during the early spring through early summer. Although there is no official record of where all these small forested wet areas are located, they’re places where activity comes alive with early season wildlife, including unique amphibians—including many frog species—that are heard long before the vernal pool may be spotted in the forest.

How to find them during the dormant season

If these vernal pools dry out later in the season, how can they be found during those times? And why would it be important to keep track of where they are in the winter?

Even if vernal pools don’t have water in them, there are a few simple ways to find them. Usually the physical depression in the forest understory can still be seen, even if there is snow cover. Since these pools collect large amounts of leaf litter, those leaves often appear to be a duller, darker color than surrounding understory litter. Even if there is snow cover that covers this leaf litter, the dormant nearby plants will give some clues. Trees and shrubs that can tolerate wet soils will often be present. These may include trees like silver maple, eastern cottonwood, black ash, or black willow. Shrubs like buttonbush, tag alder, and various viburnum species may also be found.

What’s lurking in there?

What is even more crucial than what we can see during the dormant season? The things that are lurking in all that leaf litter, and even lower in the mucky soil. Although many species like wood frogs and spotted salamanders migrate away from the pools as they begin to dry in the summer, others aren’t as mobile. The “year-round residents” that remain in the vernal pool either survive the relatively dry, dormant time by depositing drought-resistant eggs or cysts, or by burrowing into the subsurface as juveniles or adults. Crustaceans like fairy shrimp or clam shrimp do the former, where their dormant eggs—called cysts because of their hard outer covering—can wait for many years in this form until conditions are just right. Other critters, including various fingernail clams, hunker down in the muddy subsurface to wait out the drier times.

All of these creatures that stay in place during the dormant season are vulnerable to physical disturbances like forest harvesting. The Michigan Departments of Natural Resources and Environmental Quality outline harvesting guidelines that are designed to protect this fragile resource in their recently updated manual: Michigan Forestry Best Management Practices for Soil and Water Quality. The Best Management Practices (BMP) manual specifies that no equipment, trees or tree tops are to disturb the actual pool depression at any time. To protect those species that migrate to and from the pool during the active (spring and summer) season, operators are to avoid creating any deep rutting within 100 feet of the pool itself. Often this means that the ideal time for timber harvest is in the winter when the ground is completely frozen. Although tree removals are not prohibited, the BMPs recommend maintaining at least a 70 percent forest canopy cover to protect the pool from elevated temperatures or excessive drying effects.

Want to learn more?

The Michigan Vernal Pools Partnership (MVPP) is an organization dedicated to increasing awareness, understanding and protection of vernal pools through conservation, research and mapping, education and outreach, and collaboration. The MVPP explores effective ways to identify these special forest features, and how individuals and local communities can effectively conserve them. It also offers workshops and educational materials for landowners, schools, and others who are interested in learning more about them.

Learn more on how to identify and monitor vernal pools through the Vernal Pool Patrol! This effort recruits citizen scientists to help gather more information. Contact Yu Man Lee (leeyum@msu.edu) or Daria Hyde (hydeda@msu.edu) of the Michigan Natural Features Inventory to become a patrol partner in 2019!

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